Soil Sampling for the Garden
Richard W. Bosley, Mentor Ohio
Reprinted from Great Lakes Chapter News Letter.
Why Test Soil?
The only practical way to tell whether soil needs lime and fertilizer is to have the soil analyzed in a soil testing laboratory. The trouble with having your own equipment is that reagents soon become stale and the results are not too accurate. Few if any successful commercial growers do their own testing today. Frequently, just adding a little of the right type of fertilizer will make a big difference in the production of a garden. Excess fertilizer is not only wasteful but may decrease growth, burn roots and leaves and provide a point of entry for disease. Sample Properly Soil is an extremely variable mixture of mineral and organic matter. Soil from one end of the garden may have a different composition than soil from the other end. The sample sent in must be truly representative of the area in question.
When To Test
Fall, winter and early spring is a good time to test so long as you get the suggested fertilizer applied before any spring rains. Some of the fertilizer elements have low solubility and the heavy rains help carry them into the root zone. Unless you have a particular trouble spot, a testing schedule of once every three years is probably adequate for your yard.
Taking the Sample
From each area to be tested, a composite sample should be mixed which consists of twenty cores which are about 1" in diameter and 8" deep. The top soil and trash should first be removed to a depth of about 2" before taking the sample. The easiest way to take the sample is to borrow a soil sampling tube from the Extension Service. If this is not possible you could use a garden trowel to obtain a section of soil.
Most soil sampling services will make some sort of recommendation and their instructions should be followed with care. The right amount of fertilizer is fine but too much will kill. Testing Service
Most states have a Cooperative Extension Service office in each county of each state. A phone call to their office will give you more details of how to proceed but I have enclosed some data for your information. It is possible that you know of a commercial service for testing soil and there is nothing wrong with them although it will cost more but they might offer more service.
In general, rhododendrons will thrive on about one half the fertilizer levels suggested for other ornamental plants so go easy when you tip the feed bag! Some state universities offer leaf analysis also but there are no reliable standards to compare the data to. There have been suggested leaf levels published in the proceedings of the Quarterly Bulletin in the past but I have found many of these to be quite different from what I consider optimum. I am sure that there will be differences from variety to variety and that you will have to develop your own data if you choose this form of sampling. In general the leaves you would pull would be 25-30 of the first mature leaves back from the growing tip. Soil or leaf samples should be used so as not to contaminate them with foreign material that might throw the analysis off. Testing is fun because it takes some of the guess work out of growing and the results can make you look good.